1. Technical Field
This disclosure is generally related to semiconductor devices, and, more particularly, to methods of forming a metal electrode from a titanium nitride layer to prevent a leakage current, and to methods of fabricating a storage capacitor having the metal electrode.
2. Description of the Related Art
It is widely known that a memory cell of a semiconductor memory device, such as a dynamic random access memory (DRAM), includes one transistor and one storage capacitor. When the size of the memory cell is reduced in response to an increased degree of integration on the DRAM, the size of the transistor is also reduced as well as the area of the semiconductor substrate occupied by the storage capacitor. For a typical storage capacitor having a two-dimensional planar structure, the reduced area results in a decreased capacitance.
When the capacitance of the storage capacitor is decreased, the signal-to-noise ratio (S/N ratio) deteriorates, and a soft error due to an alpha (α) particle may occur. Therefore, even though a high integration level for a DRAM is desirable, the capacitance of the storage capacitor must also be sufficiently ensured.
Many methods have been proposed to achieve this goal, such as reducing the thickness of the dielectric layer, using a dielectric layer having a high dielectric constant, or enlarging an effective area of the storage capacitor.
For example, a storage capacitor having a three-dimensional structure such as a stack type structure or a trench type structure has an increased effective area compared to the planar capacitor structure. The stack type capacitor structure has been further modified to produce a cylindrical-type capacitor or a fin-type capacitor, and the technology has been developed to produce a structure for enlarging the area of the storage electrode.
When a high dielectric material such as Ta2O5, Al2O3, or HfO2 is employed for the dielectric layer of the storage capacitor, even though the dielectric constant is increased, the interface characteristics between the dielectric layer and the polycrystalline silicon used as an electrode of the storage capacitor may deteriorate. Furthermore, if the thickness of the dielectric layer is reduced, a leakage current may increase due to tunneling effects. In order to suppress the leakage current, a layer having a low dielectric constant such as a silicon oxynitride (SiON) layer may be interposed between the polycrystalline silicon electrode and the high dielectric layer, but this results in a deteriorated capacitance. Therefore, a capacitor in which a metal having a high work function such as titanium nitride (TiN) or platinum (Pt) may be used as an electrode instead of the polycrystalline silicon electrode.
For example, in the capacitor that uses Al2O3 or HfO2 as a capacitor dielectric layer, a polysilicon layer may be used as a lower electrode, and a metal layer may be used as an upper electrode. This is referred to as a metal-insulator-silicon (MIS) structure or metal-insulator-metal (MIM) structure. At this time, the Al2O3 or HfO2 dielectric layer of the capacitor may be formed using a chemical vapor deposition (CVD) method. Since carbon or hydrogen contained inside a precursor may remain, a subsequent thermal annealing process is necessary.
Furthermore, a double-layered structure such as TiN/poly-Si may be used for an upper electrode of a capacitor in order to minimize stress during a subsequent process when a metal layer composed of titanium nitride having a high work function is used.
When polycrystalline silicon is employed as an upper electrode on the hafnium oxide layer in the conventional semiconductor device as described above, a low-k dielectric material, that is, a silicon oxide layer, may be formed between the dielectric layer of the capacitor and the upper electrode of the capacitor by the reaction of the polycrystalline silicon and oxygen. A metal layer such as a titanium nitride layer may be further formed to prevent the reaction of the dielectric layer and the polycrystalline silicon.
Therefore, in the conventional method of fabricating a capacitor, the titanium nitride layer is formed on the high dielectric constant hafnium oxide layer, and a polysilicon layer doped with conductive impurities is formed on the titanium nitride layer, thereby forming an upper electrode.
Herein, the titanium nitride layer may be formed with a CVD process that uses titanium chloride (TiCl4) gas and ammonia (NH3) gas as reactant gases, as disclosed in U.S. Pat. No. 6,207,557.
First, a first titanium nitride layer is formed to a thickness of about 10 to 100 Å by a CVD method using titanium chloride (TiCl4) gas and ammonia (NH3) gas as reactant gases at a temperature of about 530 to 680° C. At this time, a degree of vacuum is maintained with about 0.2 to 0.5 torr, and a flow ratio of the ammonia (NH3) gas to the titanium chloride (TiCl4) gas is in the range of about 0.02 to 0.05.
Then, the first titanium nitride layer is thermally annealed for a predetermined time in an atmosphere of ammonia (NH3) gas, thereby forming a protecting titanium nitride layer on the surface or grain boundary of the first titanium nitride layer. The ammonia (NH3) gas flowed on the first titanium nitride layer is supplied at a flow rate of about 1000 sccm (standard cubic centimeters per minute) at a temperature of about 530 to 680° C. and with a degree of vacuum of about 3 torr.
A second titanium nitride layer is formed on the protecting titanium nitride layer by the same operations as forming the first titanium nitride layer. Herein, the second titanium nitride layer may be formed to a predetermined thickness by a CVD method using titanium chloride (TiCl4) gas and ammonia (NH3) gas as reactant gases at a temperature of about 530 to 680° C. At this time, a degree of vacuum is maintained at about 0.2 to 0.5 torr, and a flow ratio of the ammonia (NH3) gas and the titanium chloride (TiCl4) gas is in the range of about 0.02 to 0.05 sccm.
Therefore, the conventional method of fabricating the capacitor can prevent chlorine during the formation of the second titanium nitride layer from penetrating into the first titanium nitride layer by forming the protecting titanium nitride layer on the first titanium nitride layer.
However, in the conventional method of fabricating the capacitor described above, during formation of the first titanium nitride layer the titanium chloride source gas may react with the surface of the hafnium oxide layer at a high temperature to produce hafnium chloride. This may lower the performance of the resulting capacitor, thereby resulting in a decreased production yield.
Embodiments of the invention address these and other disadvantages of the conventional art.